Spatial light modulators, such as a DMD (digital micromirror device), can be used to create projectable images that are displayed in rear-projection television receivers and in consumer, business, and large-venue image projection systems. Other spatial light modulators such as LCDs (liquid crystal displays) are also used in some image projection applications. An example of a digital micromirror device is a DLP™ (“digital light processor”), manufactured by Texas Instruments, which uses an array of micromirrors with selectively controllable orientations to form a projectable image. Digital micromirror devices generally are the preferred image-forming components in projection systems requiring high illumination levels. As an example, DLP™ technology has enabled the design of small and bright projection display systems that weigh less than three pounds for a 1500 Lm output. Recently, small LED (light-emitting diode) light sources have enabled the design of one pound, pocket-sized DLP™ projectors; however, the required optical elements generally remain as a costly contributor to the size and weight of the projector. Since the light emitted by LEDs is typically in a Lambertian pattern with limited intensity, it has been difficult to couple LEDs into low-etendue spatial light modulator panels, resulting in low projection brightness levels of less than 50 Lm. To provide improved projector performance from a much smaller, lower-cost system, a DMD light modulator generally would require compact optics and a collimated light source, such as a laser diode, and would be an attractive addition to the marketplace. Such a product would preferably project an image with intensity greater than about 1000 Lm, and weigh less than about one pound.
In typical single-DMD projectors, image data comprises separate color frames corresponding to individual primary colors, typically red, green and blue, from which substantially the entire visible spectrum is derived. When solid-state or filtered colored light sources are operated in synchronization with the DMD, each color-frame image is selectively projected in sequence to a target screen for viewing by an audience.
To generate a broad-spectrum image using primary colors, monochromatic or filtered colored light sources are operated in sequence with prescribed on-times or duty cycles. When color-frames are rapidly rendered on a screen, the human eye perceives the required color hue and intensity at each pixel of the on-screen image. For example, to obtain a green-only image, a green light source would be turned on simultaneously with the DMD mirrors that spatially modulate the light beam according to the green color-frame image data. To obtain other colors, secondary colors or mixed color hues of red, green, and blue light illuminate the DMD in a sequence, at a frequency typically greater than 180 Hz. High-frequency, color-frame images are then integrated by the eyes of the audience. For example, for a white image, the duty cycle for solid-state light emitting diodes is typically 50% green, 30% red, and 20% blue light. The proportions may vary depending on the desired color temperature of the projected white image.
FIG. 1 illustrates a projection system of the prior art that uses a single DLP™ spatial light modulator with existing DLP™ optics, light source, and illumination methods. DLP™ projectors such as the illustrated projector using one DMD typically use UHP (ultra-high-pressure) arc lamps 200 and a complex set of relay optics 201 for DMD illumination. The optical path includes a condenser lens 202, a rotating color filter wheel assembly 204, relay lens 207, a spatial light modulator (DMD) 218, a projection lens 220, and a fan assembly 222. Such projection systems are complex and require substantial space for their implementation.
FIG. 2 illustrates another optical architecture of the prior art using one DMD and a TIR (total internal reflection) prism system, which generally results in an illumination light beam with an area equal to or larger than the DMD active mirror area. Again, this arrangement generally requires substantial volume for its construction. White light from high-intensity lamps such as UHP arc lamps is converted into primary wavelengths (such as red, green, and blue) by a sequence of filters arranged on a spinning disc or color wheel. This system comprises a high-intensity lamp assembly 200 (the illumination source), condenser lens 202, rotating color filter wheel assembly 204, integrator rod 206, relay lenses 208, 210, and 212, TIR prism assembly 214, DMD 218, and projection lens 220. In the one-DMD architecture illustrated in the figure, the spinning disc or color wheel in assembly 204 sequentially exposes the single DMD device to the filtered light from the high-intensity lamp to produce a colored image.
In operation, the optical architecture illustrated in FIG. 2 focuses white light from the lamp assembly 200 onto a small spot on the surface of the color filter wheel 204 by means of the condenser lens 202. Sequential color light (such as red, green, and blue) coming through the color wheel in the rotating wheel assembly 204 is integrated by the integrator rod 206 to produce a uniform light beam using multiple internal reflections in a transparent optical medium. The resulting beam is coupled to a set of relay lenses. The set of relay lenses is typically made up of a first lens 208, a second lens 210, and a third lens 212, which shape the color light beam to fit the optical aperture of TIR prism assembly 214. The sequenced colored light coupled into the TIR prism strikes a first TIR prism surface 216 at an angle greater than the critical angle of the surface and reflects off the surface onto the surface of DMD 218. Modulated light is reflected from “on-state” mirrors of DMD 218 back through the TIR prism assembly 214, and strike the TIR prism surface 216 at an angle less than the critical angle of the surface. The light therefore passes through the TIR prism surface, out of the prism assembly 214, and into the projection lens 220 which focuses the image on a screen.
FIG. 3 illustrates a light beam 305 shining on a DMD micromirror 310 at an angle to the DMD surface, typically 24 to 28 degrees. The micromirrors in the DMD are formed so that tilting a DMD micromirror, 311, plus 12 degrees (i.e., toward the illumination beam) results in a nearly normal “on-state” reflection 315 of the beam into the optical projection axis. Tilting a DMD micromirror, 312, minus 12 degrees (i.e., away from the illumination beam) results in an “off-state” beam axis 320 that is directed 48 to 52 degrees away from the optical projection axis. When the mirrors are unexcited, i.e., when they are in the “flat state,” the beam is reflected in an intermediate direction, 325. For good contrast, care should be taken to assure that “off-state” waste light does not enter the projection path.
FIG. 4 illustrates a three-DMD projector that eliminates the rotating color wheel and uses three DMDs with stationary optical elements to project a color image. In this arrangement, each DMD 418 is exposed to only one color, such as a primary color, and additional coated, reflecting prisms 435, 440, and 445 are required to separate white light into colors such as the primary colors red, green, and blue. The prisms may use dichroic coatings to filter the white light. The modulated and filtered light is then directed and recombined into the projection path. In the interest of brevity, the remaining elements in FIG. 4 and in subsequent figures with reference designations, the same as reference designations of items previously described will not be redescribed.
The illumination and projection optics as described with reference to FIGS. 1-4 generally are carefully designed, manufactured and assembled to obtain maximum brightness and performance in a DLP™ projection system. Proper image focus, color, and uniformity require numerous lenses and/or TIR prisms. Unless these elements are carefully designed, coated, and mechanically aligned, the multiple faces of the lens and prism elements can cause significant loss in contrast and light throughput. In practice, these projection systems generally are manufactured at significant cost by companies specializing in glass optics fabrication and precision optical assembly.
Generally, limitations of prior art projection systems using spatial light modulators are the use of distributed, expensive, and inefficient light sources that cannot be located directly adjacent to an image forming device, such as a digital micromirror device, with minimal space between optical components. The prior-art projection arrangements generally result in large physical volume for the projection system, which cannot be assembled on a common printed-wiring board with control, power, and sensor electronics. Severe limitations generally result for application in portable telephones, personal computer displays, game systems, and automotive/aircraft/marine displays where minimum size, weight, and cost are required. Limitations of prior art projection systems generally are also apparent in applications for portable front projectors and rear-projection televisions where image brightness, weight, and cost are critical marketing elements. Thus, what is needed in the art is a projection system using a spatial light modulator, such as a digital micromirror device, that circumvents these restrictions, utilizing smaller, lower cost, and more compact optics than can be used with UHP or incandescent light sources.